With the gradual depletion of onshore and shallow subsea subterranean hydrocarbon reservoirs, the search for additional petroleum reserves is being extended into deeper and deeper waters on the outer continental shelves of the world. As such deeper reservoirs are discovered, increasingly complex and sophisticated production systems are being developed. It is projected that soon, offshore exploration and production facilities will be required for probing depths of 6,000 feet or more. Since bottom-founded structures are generally limited to water depths of no more than about 1500 feet because of the sheer size of the structure required, other, so-called compliant structures are being developed.
One type of compliant structure receiving considerable attention is a tension leg platform (TLP). A TLP comprises a semi-submersible-type floating platform anchored to piled foundations on the sea bed through substantially vertical members or mooring lines called tension legs. The tension legs are maintained in tension at all times by ensuring that the buoyancy of the TLP exceeds its operating weight under all environmental conditions. The TLP is compliantly restrained by this mooring system against lateral offset allowing limited surge, sway and yaw. Motions in the vertical direction of heave, pitch and roll and stiffly restrained by the tension legs.
Prior TLP designs have used heavy-walled, steel tubulars for the mooring elements. These mooring elements generally comprise a plurality of interconnected short lengths of heavy-walled tubing which are assembled section by section within the corner columns of the TLP and, thus lengthened, gradually extend through the depth of the water to a bottom-founded anchoring structure. These tension legs constitute a significant weight with respect to the floating platform, a weight which must be overcome by the buoyancy of the floating structure. As an example, the world's first, and to date only, commercial tension leg platform installed in the U.K. North Sea, utilizes a plurality of tubular joints thirty feet in length having a ten-inch outer diameter and a three inch longitudinal bore. The tension legs assembled from these joints have a weight in water of about two hundred pounds per foot. In the 485-foot depth of water in which this platform is installed, the large weight of sixteen such tendons must be overcome by the buoyancy of the floating structure. It should be readily apparent that, with increasingly long mooring elements being required for a tension leg platform in deeper water, a floating structure having the necessary buoyancy to overcome these extreme weights must ultimately be so large as to be uneconomic. Further, the handling equipment for installing and retrieving the long, heavy tension legs adds large amounts of weight, expense and complexity to the tension leg platform system. Flotation systems can be attached to the legs but their long-term reliability is questionable. Furthermore, added buoyancy causes an increase in the hydrodynamic forces on the leg structure.
In addition to the weight penalty, the cost and complexity of the handling and end-connection of such tension legs is also very high. For instance, in each corner column of the floating structure, complex lowering and tensioning equipment must be provided for assembling, and extending and retrieving each of the tension legs located in that corner.
Additionally, once the tension legs are properly in position, some type of flexible joint means must be provided to allow compliant lateral movement of the platform relative to the anchor. Typical of such a structure is a cross-load bearing such as described in U.S. Pat. No. 4,391,554.
Means must also be provided on the lower end of the tension legs for interconnecting with the foundation anchors. Most of the suggested anchor connectors are of the stab-in type such as described in U.S. Pat. Nos. 4,611,953; 4,459,993; and 4,439,055. These complex structures comprise a resilient flex bearing assembly as well as some type of mechanical latch structure activated by springs and/or hydraulic forces. Obviously, the complexity and expense, as well as the potential for failure, with such structures must be taken into consideration. Another type of tendon connector which has been proposed but never used is described in British Patent No. 1,604,358. In this patent, wirerope tendons include enlarged end portions which interconnect with the anchoring means in the manner of a side-entry chain and eye connection.